1. Field of the Invention
This invention relates to ultrashort pulse laser processing of optically transparent materials, including material scribing, welding and marking.
2. Description of the Related Art
A. Cutting and Scribing
Cutting of optically transparent materials is often done with mechanical methods. Perhaps the most common method for cutting thin, flat materials is using a mechanical dicing saw. This is the standard method in the microelectronics industry for dicing silicon wafers. However, this method generates significant debris that must be managed in order to avoid parts contamination, resulting in increased overall cost of the process. In addition, the thinner wafers being used for advanced microprocessor designs tend to shatter when cut with a dicing saw.
To address these problems, current state-of-the-art processes for “scribe and cleave” material cutting use various types of lasers to scribe a surface groove on the material prior to breaking the material along this scribe. For example, a sub-picosecond laser pulses have been used to cut silicon and other semiconductor materials (H. Sawada, “Substrate cutting method,” U.S. Pat. No. 6,770,544). Also, a focused astigmatic laser beam has been used to make a single surface groove for material cutting (J. P. Sercel, “System and method for cutting using a variable astigmatic focal beam spot,” U.S. Patent Application No. 20040228004). This patent claims that by optimizing the astigmatic focusing geometry, increased processing speeds can be achieved.
To achieve a precise, high quality cleave, the groove must be of a certain minimum depth, the value of which varies by application (for example, 100-μm thick sapphire requires an approximately 15-μm deep groove for acceptable cleaving). Because the depth of the groove decreases as the scanning speed increases, the minimum depth requirement limits the maximum scanning speed, and hence the overall throughput, of a laser-scribing system. Alternative technology for material cutting uses multiphoton absorption to form a single laser-modified line feature within the bulk of a transparent target material (F. Fukuyo et al., “Laser processing method and laser processing apparatus,” U.S. Patent Application No. 20050173387). As in the case of a surface groove, there is a particular minimum size of this sub-surface feature that is required in order to yield precise, high quality cleaving of the material, which equates to a limit on the processing speed for material cutting.
A noteworthy application of “scribe and cleave” material cutting is wafer dicing for separation of individual electronic and/or optoelectronic devices. For example, sapphire wafer dicing is used in singulation of blue light emitting diodes. Wafer singulation can be accomplished with backside laser ablation, minimizing contamination of devices on the front side of the wafer (T. P. Glenn et al., “Method of singulation using laser cutting,” U.S. Pat. No. 6,399,463). Also, an assist gas can be used to aid a laser beam that dices a substrate (K. Imoto et al., “Method and apparatus for dicing a substrate,” U.S. Pat. No. 5,916,460). In addition, a wafer can be diced by first using a laser to scribe a surface groove, and then using a mechanical saw blade to complete the cutting (N. C. Peng et al., “Wafer dicing device and method,” U.S. Pat. No. 6,737,606). Such applications are generally executed in large volume and hence processing speed is of particular importance.
One process uses two different types of lasers, one of which scribes the material, and the other of which breaks the material (J. J. Xuan et al., “Combined laser-scribing and laser-breaking for shaping of brittle substrates,” U.S. Pat. No. 6,744,009). A similar process uses a first laser beam to generate a surface scribe line, and a second laser beam to crack a non-metallic material into separate pieces (D. Choo et al., “Method and apparatus for cutting a non-metallic substrate using a laser beam,” U.S. Pat. No. 6,653,210). Two different laser beams for scribing and cracking have also been used to cut a glass plate (K. You, “Apparatus for cutting glass plate,” International Patent Application No. WO 2004083133). Finally, a single laser beam has been used to scribe and crack a material by focusing the laser beam near the top surface of the material and moving the focus down through the material to near the bottom surface while providing relative lateral motion between the focused laser beam and the target material (J. J. Xuan et al., “Method for laser-scribing brittle substrates and apparatus therefor,” U.S. Pat. No. 6,787,732).
B. Material Joining
The joining of two or more optically transparent materials, such as glasses and plastics, is useful for applications in various industries. The construction of any type of device in which optical transparency allows or supplements functionality, or otherwise results in additional value (e.g. aesthetic), could benefit from such a joining process. One example is hermetic sealing of components where visual inspection is needed (e.g. telecommunications and biomedical industries).
In some applications, conventional joining processes (e.g. adhesives, mechanical joining) are inadequate. For example, many adhesives might prove non-biocompatible in the case of biomedical implant devices. For other devices, the adhesion simply may not be strong enough for the particular application (e.g. high-pressure seals). For such demands, laser welding offers an ideal solution.
In microfluidic systems, the sealing of individual, closely spaced paths relative to each other with a cap piece that covers the entire device would be desirable. Strong, tightly sealing joints can be difficult to make with other methods due to the small contact region between the different microfluidic paths. Laser welding can precisely position the bonded regions between these microfluidic paths and provide a tight seal.
The current state-of-the-art technology for laser welding of transparent materials consists of:                (1) use of a CO2 laser, the wavelength (˜10 μm) of which is linearly absorbed by many optically-transparent materials, or        (2) introduction of an additional material at the interface of the transparent materials, which is specially designed to absorb the laser radiation, thereby causing heating, melting, and fusing of the materials.        
Both of these methods are limited in their functionality and/or costly in their implementation.
A pulsed CO2 slab laser has been used to weld Pyrex to Pyrex, and to bond polyimide and polyurethane to titanium and stainless steel (H. J. Herfurth et al., “Joining Challenges in the Packaging of BioMEMS,” Proceedings of ICALEO 2004). Also, fused quartz and other refractory materials have been welded with a 10.6 μm CO2 laser (M. S. Piltch et al., “Laser welding of fused quartz,” U.S. Pat. No. 6,576,863). The use of such CO2 lasers does not allow welding by focusing the beam through a thick top layer material, since the laser radiation is absorbed before it can reach the interface. An additional disadvantage is that the large wavelength does not allow focusing the beam to a small spot, thereby limiting its usefulness for creating small weld features on micron scales.
Alternatively an absorbing layer that is transparent to the human eye can be placed between two materials to be welded, such as polyamide and acrylic (V. A. Kagan et al., “Advantages of Clearweld Technology for Polyamides,” Proceedings ICALEO 2002). A diode laser with line focusing is then used for welding (T. Klotzbuecher et al., “Microclear—A Novel Method for Diode Laser Welding of Transparent Micro Structured Polymer Chips,” Proceedings of ICALEO 2004). The dye layer is specially designed to absorb the laser's wavelength (R. A. Sallavanti et al., “Visibly transparent dyes for through-transmission laser welding,” U.S. Pat. No. 6,656,315).
One welding process for bonding glass to glass or metal employs a laser beam to melt a glass solder between the surfaces to be welded (M. Klockhaus et al., “Method for welding the surfaces of materials,” U.S. Pat. No. 6,501,044). Also, two fibers can be welded together by using an intermediary layer that is linearly absorbent to the laser wavelength (M. K. Goldstein, “Photon welding optical fiber with ultra violet (UV) and visible source,” U.S. Pat. No. 6,663,297). Similarly, a fiber with a plastic jacket can be laser-welded to a plastic ferrule by inserting an absorbing intermediary layer (K. M. Pregitzer, “Method of attaching a fiber optic connector,” U.S. Pat. No. 6,804,439).
The use of an additional layer of an absorbing material has significant drawbacks. The most obvious is the cost of purchasing or fabricating a material that is appropriate for the process. A potentially more costly issue is the increase in processing time associated with incorporating this additional material into the manufacturing process. Such costs would be expected to rise dramatically as the size of the desired weld region becomes increasingly small, as would be the case with biomedical implant devices. Another disadvantage of using an intermediate, light-absorbing layer is that this layer may introduce contaminants into the area to be sealed. In the case of a microfluidic system, the light-absorbing layer would be in direct contact with the fluid flowing through the channel.
One method for welding a transparent material to an absorbing material is called through-transmission welding. In this method a laser beam is focused through a transparent material and onto an absorbing material, resulting in welding of the two materials (W. P. Barnes, “Low expansion laser welding arrangement,” U.S. Pat. No. 4,424,435). This method has been used to weld plastics by directing polychromatic radiation through a top transparent layer and focusing the radiation onto a bottom absorbing layer (R. A. Grimm, “Plastic joining method,” U.S. Pat. No. 5,840,147; R. A. Grimm, “Joining method,” U.S. Pat. No. 5,843,265). In another example of this method, a black molded material that is transparent to the laser wavelength is welded to an adjacent material or via an added welding assist material that absorbs the laser wavelength (F. Reil, “Thermoplastic molding composition and its use for laser welding,” U.S. Pat. No. 6,759,458). Similarly, another method uses at least two diode lasers in conjunction with a projection mask to weld two materials, at least one of which is absorbent of the laser wavelength (J. Chen et al., “Method and a device for heating at least two elements by means of laser beams of high energy density,” U.S. Pat. No. 6,417,481).
Another laser welding method performs successive scans of a laser beam over the interface between two materials to incrementally heat the materials until melting and welding occurs (J. Korte, “Method and apparatus for welding,” U.S. Pat. No. 6,444,946). In this patent one material is transparent, while the other material is absorbent to the laser wavelength. Finally, one method uses ultraviolet, laser, X-ray, and synchrotron radiation to melt two pieces of material, and then brings them into contact in order to form a weld (A. Neyer et al., “Method for linking two plastic work pieces without using foreign matter,” U.S. Pat. No. 6,838,156).
Laser welding is disclosed for hermetic sealing of organic light emitting diodes where there is at least one layer of organic material between two substrates (“Method of fabrication of hermitically sealed glass package”, U.S. Patent Application Publication 20050199599).
Tamaki et al. discuss the use of 130-fs laser pulses at 1 kHz to bond transparent material in “Welding of Transparent Materials Using Femtosecond Laser Pulses”, Japanese Journal of Applied Physics, Vol. 44, No. 22, 2005. However, the material interaction of low repetition rate ultrashort pulses (kHz) is known to be distinctly different compared to high repetition rate ultrashort pulses (MHz) due to electron-phonon coupling time constants and accumulation effects.
C. Sub-Surface Marking
The patterning of sub-surface marks in glass has been adapted by artists to create 2-D portraits and 3-D sculptural works. These marks are designed to be strongly visible under a wide range of conditions without requiring external illumination.
Tightly focusing energy below the surface of optically transparent materials can produce visible, radially propagating micro-cracks. Long-pulse lasers are commonly used to create these marks. Several patents discuss variation of the size and density of these radial cracks to control the visibility of the subsequent pattern (U.S. Pat. Nos. 6,333,486, 6,734,389, 6,509,548, 7,060,933).
The visibility of the mark can be controlled by the crack density around the central laser spot, rather than just the size of the mark (U.S. Pat. No. 6,417,485, “Method and laser system controlling breakdown process development and space structure of laser radiation for production of high quality laser-induced damage images”).
U.S. Pat. No. 6,426,480 (“Method and laser system for production of high quality single-layer laser-induced damage portraits inside transparent material”) uses a single layer of smooth marks where brightness is controlled by the spot density.
Increasing the pulse duration of the writing laser light will increase the brightness of the mark (U.S. Pat. No. 6,720,521, “A method for generating an area of laser-induced damage inside a transparent material by controlling a special structure of a laser irradiation”).